At present, the clubs used by golfers at the tee, or starting area, to drive the ball over long distances are called woods. Woods were originally constructed, in their entirety, of wood materials, such as persimmon or other similar varieties. These clubs are still prized by many golfers, but their performance tolerances are narrow, because of the low density of the material used and its homogeneous distribution behind the impact surface of the club head.
To remedy this disadvantage, the "wood-metal" club was created, a club which reproduces the shape of the wood, but whose head is made entirely of steel. Because of the high density of the material used and the necessary weight constraints, wood-metal clubs generally incorporate a hollow steel head, normally manufactured using the lost wax casting process. In this type of construction, the weight, which is mainly distributed on the periphery of the impact surface, imparts to the club a tolerance which is clearly greater than that of conventional woods. This tolerance applies both to the angle at which the ball leaves the club, an angle which determines the vertical trajectory of the ball, and to the deviation of the ball, i.e., to its lateral trajectory.
A first disadvantage of wood-metal clubs lies in the unpleasant sensations experienced by the golfer at the time of impact and caused by the contact of the metal face with the ball.
Another problem arises from the fact that the stiffness of the steel impact surface, whose thickness must be sufficient to absorb the shock of the blow, is not optimal. It can be demonstrated that reduced stiffness of the surface increases the speed of restitution, thus increasing the distance of flight. It can be shown that the stiffness of the surface depends on the thickness of the impact surface and on the modulus of elasticity of the material. For a given modulus of elasticity, reduced stiffness is, therefore, directly linked to a reduction of the thickness of this surface. It is today apparent that the optimal degree of stiffness of the impact surface of a wood-metal club corresponds to an excessive thinness, i.e., less than 3 millimeters, thus leading to irreversible deformation.
According to a well-known arrangement, the hitting surface of wood-type club heads is not planar, as is the casing for irons, but is rounded in both vertical and horizontal cross-sections. The advantage of these curvatures is that they correct the trajectory of the ball, more particularly when the point of impact of the blow is not located exactly in the center of the surface. Because of these curvatures and of present-day design of conventional clubs, their performance when used to strike the ball is uneven, a fact which golfers can only complain about.